Sihanouk’s government was recognized as the sole legitimate authority within Cambodia at the Geneva Conference convened in 1954 to reach a political settlement to the First Indochina War (the Geneva Accords). That decision prevented the Viet Minh from gaining any regional power in Cambodia, as they did in Laos.

While Democrats and communists alike recognized Sihanouk’s role in gaining Cambodia’s independence, they opposed his increasing authoritarianism. Sihanouk abdicated the throne in March 1955 in favor of his father, Norodom Suramarit, and formed a mass political movement, the Sangkum Reastr Niyum (“People’s Socialist Community”), whose members were forbidden to belong to other political parties. The effect of the move was to draw thousands of people away from the Democrats, who had expected to win the national elections scheduled for later in the year. When the elections took place, amid widely reported abuses by Sihanouk’s police, the Sangkum won every seat in the National Assembly. Sihanouk became the central figure in Cambodian politics from then until his overthrow in 1970, as prime minister and—after his father’s death in 1960, when no new monarch was named—as head of state. Overt political life was strictly controlled by the prince, his colleagues, and the police; Cambodian communists, a marginal group of fewer than a thousand members, operated clandestinely and enjoyed little success. In 1963 Saloth Sar, a schoolteacher who was also secretary of the Communist Party, fled Phnom Penh and took refuge in the forests along the Vietnamese border; from there he built the organization that later would be known as the Khmer Rouge.

Sihanouk was widely revered in Cambodia until the late 1960s, when opposition to his rule intensified. He saw Thailand and what was then South Vietnam as the greatest threats to Cambodia’s survival. Those two countries were allied with the United States, which the prince disliked. At the same time, Sihanouk feared the eventual success of the Vietnamese communists in their war against South Vietnam and the United States, and he dreaded the prospect of a unified Vietnam under communist control. To gain some freedom to maneuver, he proclaimed a policy of neutrality in international affairs. Sihanouk broke off relations with the United States in 1965, convinced of American involvement in two South Vietnamese-backed plots against the Cambodian state in 1959 and encouraged in his anti-Americanism by the French president, Charles de Gaulle, whom he idolized. Soon afterward he concluded secret agreements with the Vietnamese communists, who were allowed to station troops on Cambodian territory in outlying districts as long as they did not interfere with Cambodian civilians. The secret agreement protected Sihanouk’s army from attacks by the Vietnamese but compromised his neutralist policies. After 1965, when the war in Vietnam intensified, he also edged toward an alliance with China.

Cambodia’s internal politics after 1965 developed in a complex fashion. Elections in 1966, the first since 1951 not to be stage-managed by the prince, brought in a majority of National Assembly members who owed little or nothing to Sihanouk himself. Although the prince was still a revered figure among the rural populace, he became increasingly unpopular with the educated elite. Conservatives resented his break with the United States and his seemingly pro-communist foreign policy, while Cambodian radicals opposed his internal policies, which were economically conservative and intolerant of dissent. A rebellion in Bătdâmbâng province in 1967, manipulated by local communists, convinced the prince that the greatest threat to his regime came from the radical sector, and without hesitation he began using severe measures—including imprisonment without trial, assassinations, and the burning of villages—to impose his will.

By 1969 Sihanouk’s grip on Cambodian politics had loosened, and conflict between his army and communist guerrillas, especially in the northeast, had increased. Some anticommunist ministers led by Prince Sirik Matak and General Lon Nol plotted to depose Sihanouk, whose credibility with radicals had evaporated following his renewal of diplomatic relations with the United States. Sihanouk’s elaborate policy of juggling major powers against each other had failed. Matak and Lon Nol worked closely with anticommunists in South Vietnam, including Son Ngoc Thanh, whose Khmer Serei movement had gained recruits among the Khmer-speaking minority in Vietnam.